Making Cores
The core is a sand shape inserted into the mold to produce the internal features of the part such as holes or internal passages. Core print is the region added to the pattern, core, or mold that is used to locate and support the core within the mold. A riser is an extra void created in the mold to contain excessive molten material. The purpose of this is to help feed the molten metal to the mold cavity. As the metal solidifies and shrinks it thereby helps prevent voids in the main casting. Extra metal is removed in the finishing process.
See the article below for an in-depth discussion of the sand casting process.